Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

The following schedule provides information related to our stock repurchase program for each of the three years ended December 31, 2017. During 2017, 2,960,843 shares ($322.2 million) were repurchased pursuant to the terms of our stock repurchase program, 305,278 shares ($34.2 million in the aggregate) were repurchased in connection with the income tax withholding obligations resulting from the exercise of stock options and the vesting of restricted stock grants and 10,791 shares were repurchased as a result of forfeited restricted shares. During 2016, 2,512,592 shares ($289.9 million) were repurchased pursuant to the terms of our stock repurchase program, 468,228 shares ($57.0 million in the aggregate) were repurchased in connection with the income tax withholding obligations resulting from the exercise of stock options and the vesting of restricted stock grants and 2,500 shares were repurchased as a result of forfeited restricted shares. During 2015, 1,326,207 shares ($166.2 million) were repurchased pursuant to the terms of our stock repurchase program and 493,296 shares ($58.0 million in the aggregate) were repurchased in connection with the income tax withholding obligations resulting from the exercise of stock options and the vesting of restricted stock grants.

	Additional dollars authorized for repurchase (in thousands)		Total number of shares purchased (a.)		Total number of shares cancelled		Average price paid per share for forfeited restricted shares		Total number of shares purchased as part of publicly announced programs		Average price paid per share for shares purchased as part of publicly announced program		Aggregate purchase price paid (in thousands)		Aggregate purchase price paid for shares purchased as part of publicly announced program		Maximum number of dollars that may yet be purchased under the program (in thousands)
Balance as of January 1, 2015																	$	342,050
2015	$	—		1,819,503		—	N/A			1,326,207	$	125.34	$	224,260	$	166,222	$	175,828
2016	$	400,000		2,983,320		2,500	$	0.01		2,512,592	$	115.39	$	346,890	$	289,937	$	285,891
2017	$	400,000		3,266,121		10,791	$	0.01		2,960,843	$	108.83	$	356,413	$	322,231	$	363,660
Total for three year period ended December 31, 2017	$	800,000		8,068,944		13,291	$	0.01		6,799,642	$	114.48	$	927,563	$	778,390

(a)

Includes 10,791 and 2,500 of restricted shares that were forfeited by former employees pursuant to the terms of our restricted stock purchase plan during 2017 and 2016, respectively.

Weighted-Average Assumptions Fair Value of Option Grants

The weighted-average assumptions reflected below were based upon twenty-seven option grants for the five-year period ending December 31, 2017, twenty-seven option grants for the five-year period ending December 31, 2016 and twenty-five option grants for the five-year period ending December 31, 2015.

Year Ended December 31,	2017			2016			2015
Volatility		28	%		31	%		33	%
Interest rate		1	%		1	%		1	%
Expected life (years)		3.4			3.4			3.4
Forfeiture rate		10	%		10	%		10	%
Dividend yield		0.4	%		0.4	%		0.4	%

Stock Option Activity

The table below summarizes our stock option activity during each of the last three years:

Outstanding Options	Number of Shares			Average Option Price		Range (High-Low)
Balance, January 1, 2015		7,897,451		$	57.29	$102.21-$36.95
Granted		3,039,350		$	117.70	$142.43-$108.29
Exercised		(2,256,454	)	$	48.97	$102.21-$36.95
Cancelled		(280,164	)	$	83.63	$134.70-$36.95
Balance, January 1, 2016		8,400,183		$	80.50	$142.43-$36.95
Granted		2,945,550		$	118.72	$138.00-$107.39
Exercised		(2,162,850	)	$	53.02	$117.29-$36.95
Cancelled		(412,750	)	$	103.01	$130.32-$36.95
Balance, January 1, 2017		8,770,133		$	99.06	$142.43-$36.95
Granted		3,061,725		$	124.38	$124.56-$110.15
Exercised		(1,734,409	)	$	64.41	$118.62-$36.95
Cancelled		(457,500	)	$	118.65	$142.43-$53.38
Balance, December 31, 2017		9,639,949		$	112.40	$138.00-$53.38
Outstanding options vested and exercisable as of December 31, 2017		2,869,346		$	100.51	$138.00-$53.38

Information about Unvested Options

The following table provides information about unvested options for the year December 31, 2017:

	Shares			Weighted Average Grant Date Fair Value
Unvested options as of January 1, 2017		6,695,266		$	20.94
Granted		3,061,725		$	27.05
Vested		(2,555,013	)	$	19.21
Cancelled		(431,375	)	$	24.02
Unvested options as of December 31, 2017		6,770,603		$	24.16

Information about all Outstanding Options

The following table provides information regarding all options outstanding at December 31, 2017:

	Options Outstanding		Options Exercisable
Number of options outstanding		9,639,949		2,869,346
Weighted average exercise price	$	112.40	$	100.51
Aggregate intrinsic value as of December 31, 2017	$	65,133,533	$	44,588,121
Weighted average remaining contractual life		2.9		2.0

Weighted Average Remaining Contractual Life for Options Outstanding and Weighted Average Exercise Price Per Share for Exercisable Options

The weighted average remaining contractual life for options outstanding and weighted average exercise price per share for exercisable options at December 31, 2017 were as follows:

Exercise Price	Options Outstanding		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Life (in Years)		Exercisable Options		Weighted Average Exercise Price Per Share		Expected to Vest Options (a)		Weighted Average Exercise Price Per Share
	Shares						Shares				Shares
$53.38 – $79.79		1,752,085	$	76.37		1.1		1,173,671	$	75.48		565,589	$	78.17
$96.98 – $117.29		2,384,488		117.06		2.2		1,070,813		117.12		1,152,681		117.05
$118.60 – $124.44		2,583,401		118.65		3.2		605,862		118.65		1,549,346		118.65
$124.56 – $138.00		2,919,975		124.69		4.2		19,000		131.62		1,763,506		124.67
Total		9,639,949	$	112.40		2.9		2,869,346	$	100.51		5,031,122	$	118.17

(a)	Assumes a weighted average forfeiture rate of 9.64%.